This invention generally relates to alloys that are highly resistant to corrosion, and more particularly to MCr alloys, where M is a metal such as Fe, Ni, Co, or alloys of these metals.
Numerous alloys have been specifically developed for their resistance to corrosion. Such alloys are employed, for example, in turbine engines used in high temperature aerospace applications, and in many other highly corrosive environments. Often, articles made from these alloys are covered with a coating to improve the resistance of the article to corrosion. One class of such alloys is referred to as MCr alloys, where M is a metal such as Fe, Co, Ni, and occassionally alloys of these metals. These alloys also are frequently utilized as coating, and commonly they further include Al and small amounts of Y or an equivalent reactive metal.
In most of these alloys, the Cr content is greater than 15 percent by weight, with some of the alloys containing Cr in the range of 25 to 40 percent by weight. Cr is a relatively expensive material, however. Also, more than 90 percent of the Cr used in the United States is imported, and the availability and precise cost of foreign Cr are often very unstable. Because of these disadvantages, and other disadvantages associated with using imported materials, efforts have been made to provide suitable corrosion-resistant alloys that do not include as much Cr.
It is known that the Cr content of these alloys can be reduced without affecting the resistance of the alloy to corrosion by substituting sufficiently high concentrations of Si. However, the presence of these high concentrations of Si increases the brittleness of the alloys considerably, resulting in materials that are unsuitable for many, if not most, of the applications in which the MCr alloys are used.